Ring Formation Mechanism of C4H3 Radical and Acetylene in Soot Precursor Formation
This study systematically investigates the cyclization reaction mechanisms between n-C4H3 (1-buten-3-yn-1-yl) and i-C4H3 (2-buten-3-yn-1-yl) radicals with acetylene (C2H2) using density functional theory (DFT) and transition state theory (TST). The results reveal that the reaction of n-C4H3 with acetylene proceeds via a radical chain mechanism through an addition-cyclization pathway, yielding phenyl (six-membered ring), fulvenyl (five-membered ring), and four-membered ring intermediates. The product formation rates follow the order: fulvenyl (five-membered ring) > phenyl (six-membered ring) > four-membered ring. For i-C4H3, the intermediate structures depend on the carbon position of i-C4H3 where acetylene addition occurs: addition at the C2 position predominantly generates fulvenyl (five-membered ring) as the primary product, whereas addition at the C4 position may lead to phenyl (six-membered ring), fulvenyl (five-membered ring), or four-membered ring intermediates, with the four-membered ring forming most rapidly and the six-membered ring the slowest. Theoretical analyses demonstrate that the selectivity of reaction pathways is primarily governed by structural differences between the isomers. This work provides atomic-scale insights into the cyclization processes between acetylene and C4H3 species, establishing a foundation for refining models of soot precursor formation.